This invention concerns a method for the operation of a gas turbine cycle whose art is presented in the introduction of the first claim.
It is already known that the same operational conditions are, in certain cases, reached by a gas turbine cycle of the art described in claim 1, whereby water and/or steam is heated with flue gases from the expander of the said gas turbine cycle during operation.
In traditional gas turbines, air is used as the working fluid. The intake air is compressed to high pressure after which fuel is mixed with the compressed air. This mixture is then combusted in a combustion chamber and expanded through a turbine. This turbine produces work, a part of which is used to drive the compressor, while the excess is extracted as useful work. The efficiency of this process can be described as the ratio between the useful work and the energy content of the fuel utilised. In traditional gas turbine cycles, the combustion process occurs with a large amount of excess air; consequently, a majority of the intake air passes through the cycle without its oxygen content being utilised.
It is already known that gas turbine power plants may be designed as a so-called humidified cycle. In a humidified cycle, water is mixed with the working fluid. This water may be injected as steam and/or hot water may be used to humidify a part of, or the entire compressed air flow. Steam is usually generated in a heat recovery steam generator that utilises the heat contained in the flue gases from the turbine.
However, in practice, a traditional “dry” gas turbine, which is optimised for operation with dry air as the working fluid, exhibits unacceptable operational characteristics if large amounts of water are injected into the working fluid. This is because the gas flow through the turbine expander is significantly greater then the flow rate for which it is dimensioned for.
Traditional gas turbines can, therefore, not be operated as humidified cycles. Of those few gas turbines that do utilise steam injection, the amount of steam that may be injected into the working fluid is limited, and consequently not all the steam available from the heat recovery steam generator may be utilised.
If humidified cycles are to be realised, a large and costly re-design of the gas turbine machinery is necessary to accommodate the changed flow distribution and imbalance between the volume capacity of the compressor and turbine.
In addition, traditional gas turbines often cool the hot temperature regions of the turbine by utilising cold cooling air from the compressor; thus, the compressor has a certain over-capacity in the form of this cooling air. In humidified cycles, however, it is significantly advantageous to utilise steam for cooling instead of compressed air. This further increases the degree to which the gas turbine must be re-designed, as the imbalance between the volume capacity of the compressor and turbine becomes even larger when steam cooling is introduced. Thereby, extra measures must be implemented to adapt existing, air-cooled gas turbines to ones with more effective steam cooling.
While it is already known that gas turbine cycles may be utilised and operated as described in the introduction of claim 1, one further goal with the current invention is to remove the above-mentioned re-design problems associated with humidified cycles and to propose configurations that offer especially high efficiencies and favourable operating conditions.
An additional goal of the invention is to introduce a method that enables the effective operation of the second gas turbine group in such a way that a near-optimal operation of the first gas turbine group in a humidified cycle is obtained, despite the fact that the first gas turbine group has a design which in essence is optimised for a dry cycle.